Jeffrey T. Pietras1, Alan R. Carroll1, Reuben C. Johnson1
(1) UW-Madison, Madison, WI
ABSTRACT: Nonmarine Sequence Stratigraphic Interpretation of the Wilkins Peak Member, Eocene Green River Formation, Wyoming
This study provides detailed documentation of a basin-scale stratigraphic cross-section of the Wilkins Peak Member of the Green River Formation, based on outcrop description and field gamma-ray profiles. Excellent exposures occur along a 100 km transition from alluvial and lake-marginal facies in the north, to fine-grained basin-center deposits in the south. Numerous tuffs and several regionally extensive arkose beds facilitate lateral correlation of reservoir elements deposited in this underfilled lake basin. The base of the Wilkins Peak is marked by a sequence boundary formed during renewed uplift of the Wind River Range along the Continental Fault. This uplift diverted a major river drainage away from the basin, and contributed to a shift from balanced
-fill to underfilled conditions. The lower Wilkins Peak Member thickens southward, as alluvial fan deposits grade laterally into lake-plain, lacustrine carbonate, and evaporite facies. Numerous meter-scale packages record repeated lake expansion-contraction cycles that are typified by calcareous muds and oil shales that coarsen-upward to siltstones and locally very-fine sand. These cycles are stacked between arkosic sandstones derived from the eastern and southern basin margins, interpreted as sheetflood deposits. These beds were deposited during transgressive phases of Lake Gosiute and overlie exposure surfaces. The upper Wilkins Peak Member is relatively uniform in thickness, suggesting decreased subsidence in the southern depocenter. Lake expansion-contraction cycles thicken upward and contain more oil shale beds than below, recording an increase in runoff to the basin. Finally, the Wilkins Peak Member grades upward into oil shale-rich deposits of the
balanced
-filled Laney Member.
AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado